The present invention relates to improvements in an apparatus for reducing the pressure in the ullage of a volatile chemical tank to prevent fugitive emissions of polluting VOC vapors and to aid in the recovery of the chemical for use.
Storage tanks and piping for volatile organic chemicals (VOC's) have an area above the liquid known as the ullage, in which air and volatile chemical vapors reside. Various pressure changes can occur in the tank due to diurnal temperature changes, periodic refilling and the like, leading to an overpressure in the underground tank. These overpressures are of concern, since the result can be fugitive emissions of pollutants to the atmosphere. If pressurized, the air and volatile chemical vapors will have a tendency to leak out of any hole in the tank or piping of the system, thus allowing the release of the polluting volatile chemical vapor to the atmosphere, which is undesirable from an environmental protection point of view. It is also undesirable economically, because the material lost to the atmosphere has commercial usefulness.
The present applicant together with two co-inventors addressed the problem of fugitive emissions of fuel vapors in copending application Ser. No. 08/153,528 filed Nov. 16, 1993, now U.S. Pat. No. 5,464,466. The entire disclosure of that application is incorporated herein by reference. That prior application discloses and claims a fuel storage tank vent filter system in which vapors from the underground tanks are directed to a chamber having a membrane such as the fractionating membrane developed by GKSS-Forschungszentrum Geesthacht GmbH of Germany and described in "Operating Experiences with Membrane Systems in Gasoline Tank Farms," presented by K. Ohlrogge at the 1991 Ninth Annual Membrane Technology/Planning Conference in Newton, Mass., Oct. 4-6, 1991, or "Volatile Organic Compound Control Technology by Means of Membranes," presented by K. Ohlrogge at the 1993 Eleventh Annual Membrane Technology/Separation Planning Conference in Newton, Mass. on Oct. 11-13, 1993, the disclosures of which are hereby incorporated by reference. The membrane permits transmission of VOC's through it (permeate) in preference differentially to atmospheric vapors. That system calls for a pump to be arranged to draw the pollutants through the membrane and redirect them to the underground tank, permitting air (retentate) to be released to the vent pipe of the service station tank arrangement. Similar membranes are available from Membrane Technology and Research, Inc., Menlo Park, Calif.
While the apparatus described in my earlier application works well, it does not work optimally, and the vapors released out the vent include greater amounts of VOC vapors than are necessary or desirable. Also, my prior application does not specifically address itself to non-fuel volatile chemicals such as dry cleaning fluids and organic solvents.
Accordingly, there is a need in the art for refinement of a vent scrubber system to enhance the recovery of the VOC's in order to minimize their release to the atmosphere.